In the recent years, the market of optical disks exclusively used for regeneration such as DVD-ROM, DVD-Video, etc. is expanding. Furthermore, overwriting type DVD such as 4.7 GB DVD-RAM, 4.7 GB DVD-RW, etc. are put into market, and the market thereof is rapidly expanding as image-recording media capable of replacing the back-up media for computer, or VTR.
The hitherto known DVD-RAM has a 7-layered structure, as described in, for example, Proceedings of SPIE—The international Society for Optical Engineering, Volume 3401, pp. 24–32, 1998. That is to say, it has a 7-layered structure comprising a successive lamination of substrate 1, first interference layer 2, first interface layer 3, recording layer 4, second interface layer 5, second interference layer 6, absorption compensation layer 7 and heart sink layer (Al alloy) 8, as shown in FIG. 10. In this structure, a ZnS—SiO2 type material is used as the first and second interference layers; a Ge—N or Cr2O3 type material higher in melting point and chemical stability than the ZnS—SiO2 type materials is used as the first and second interface layers; and Ge2Sb2Te5 or the like is used as the recording layer 4.
A number of researches have been conducted so far for the purpose of discovering a thermally and optically excellent interference layer material. For example, in JP-A-63-103453 is disclosed an optical information recording member having an interference layer comprising at least one material selected from ZnS and ZnSe and at least one material selected from SiO2, GeO2, SnO2, TeO2, ZrO2, Al2O3, Y2O3 and Ta2O5. It is mentioned in this patent gazette that the laser power necessary for crystallization can be lessened by incorporating 10–30 mol % of SiO2 into the interference layer.
In JP-A-10-275361 is disclosed an optical information recording member having an interference layer comprising at least one material selected from ZnS and ZnSe and at least one material selected from SiO2, GeO2, SnO2, TeO2, ZrO2, Al2O3, Y2O3 and Ta2O5. It is mentioned in that patent gazette that, when the metallic elements constituting the above-mentioned oxides present in the interference layer occupy 30–65 atomic % based on the sum of Zn and the oxides-constituting metallic elements which is taken as 100 atomic %, the laser powder necessary for crystallization is small and there can be obtained an optical information recording member excellent in repeated overwrite characteristics, modulation and flexibility in the optical design.
Furthermore, in JP-A-10-302308 is described a protective layer contacted with the recording layer, which is obtained by adding an oxide such as SiO2, GeO2, SnO2, In2O3 or the like or a nitride such as Si3N4 or the like to a crystalline metal-chalcogen compound such as ZnS, ZnSe, ZnTe, PbS, PbTe or the like.